If you are putting a Gaming rig together, then my advice is to go with the 2500K, since Hyper Threading does not enhance or affect gaming performance, save the extra $100 over the 2600k.

If you do video editing, encoding, rendering, and use programs that do take advantage of Hyper Threading, then the 2600K is an excellent choice and performer.

At this point, I cannot recommend anyone putting a new rig together to go with the x58 platform, since Sandy Bridge processors are great performers, they use less power, and the K processors overclock higher on air.

Existing X58 Users

If you're an existing X58 user, you still have an excellent setup, Sandy Bridge matches, and often exceeds the current performance, but the increase is minimal in the grander scheme of things, and it's not worth the upgrade hassle. You are much better off waiting for Ivy Bridge since 6 - 8 Core processors will be available, and THIS is where we will experience noticeable gains.

LGA 1155 will support Quad Core Ivy Bridge processors, it MAY support a single 6 Core with HT disabled, however, the much improved 6 - 8 core processors will only be available on the Enthusiast socket LGA 1365.

Who Should Upgrade to Sandy Bridge?

If the thought of having a processor @ 4.5 GHZ - 5 GHZ on air appeals to you, and you're coming from Socket 775, AM2/AM3 rigs, I can confidently say that you will gain the most from your upgrade to Sandy Bridge compared to 1366 and some 1156 users would. Especially if you can sell your hardware and get a reasonable price for it, your upgrade path will be worthwhile.

If You Are Upgrading to Sandy Bridge Be Sure to Post Your Benchmarks and Results Here!